The administration of neutralizing antibodies before or immediately after HIV-1 challenge is the only identified therapy that consistently prevents infection in primate models of HIV-1. Only a limited number of viral envelope regions are important for viral neutralization, access to the most conserved regions is camouflaged or restricted on the native virion, and antibodies which neutralize will have very unusual physical structure. Thus, the most potent neutralizing antibodies are rarely made during natural infection and are not elicited by vaccination. Antibodies can also contribute to control of infections by mechanisms that do not require neutralization as measured by viral neutralization assays. Antibodies may even promote infection when mechanisms, such as complement deposition, contribute to viral binding and spread or immune dysfunction. We propose, given the structural constraints of the virus and the biological limitations of the humoral immune response, that a better understanding of the full potential of antibodies to prevent viral infection is needed. We propose the following two hypotheses (1) Antibodies can mediate a variety of specific functional activities in addition to neutralization that can prevent or enhance HIV-1 infection; moreover, antibodies can be modified to improve the outcomes of these processes; and (2) that neutralizing antibodies can be molecularly modified to improve neutralization. Antibodies have been selected for study based on epitope, neutralization breadth of binding and availability. Antibodies will be tested for complement-mediated enhancement of infection, neutralization in the presence of complement, transmission of opsonized virus by dendritic cells and B cells and B cell function. Antibodies with complement related effects will be modified to eliminate complement fixation and molecularly modified antibodies will be tested in these assays with controls for anti-HIV activity and ADCC. Isotype switched antibodies will also be generated, including IgA antibodies to study function. The isotype switched constructs will be tested for neutralization, virion binding, ADCC, neutrophil mediated viral destruction, prevention of transmission of HIV in trans, and prevention of trancytosis. To improve neutralization, single chain antibodies will be constructed for selected neutralizing antibodies to improve access to neutralizing epitopes and homo and hetero-multimers of antibodies will be constructed to study and improve antibody function. All neutralization constructs will be tested for virion binding, dynamic virion binding, neutralization, and inhibition of viral infection, replication and transmission to correlate binding, function, and structure. Long-term objectives will be to select constructs from in vitro testing for further testing in non-human primate models of prevention and therapeutics.